The diaphragm of an electret used in a microphone vibrates in response to an acoustic wave signal impinging upon the diaphragm; that vibration varies the spacing between the diaphragm and a backplate. The diaphragm and backplate comprise the electrodes of the electret. One of the electrodes is electrostatically charged to establish a voltage differential between them. The resulting variation in capacitance between the electrodes created by movements of the diaphragm enables the electret to generate an electrical signal representative of the impinging acoustic signal. In a sound reproducer, the process works in reverse. An electrical signal applied across the diaphragm and backplate electrodes of the electret vibrates the diaphragm to generate an acoustic signal.
Electret transducers can be made quite tiny in size, and hence have frequently found use in hearing aids, particularly in-the-ear hearing aids. The maximum dimension of an electret transducer for a hearing aid, particularly a microphone, may be of the order of 0.125 inch. Indeed, a hearing aid may incorporate two electret transducers; a electret microphone is used to convert acoustic signals to electrical signals, which are then amplified and applied to an electret sound reproducer (often called a "receiver") for reconversion to an acoustic signal fed into the user's ear canal.
But a hearing aid presents a decidedly adverse environment for an electroacoustic transducer of any kind. Temperature and moisture conditions vary materially. Aging affects virtually any transducer; in an electret, in particular, the voltage differential between the diaphragm and backplate electrodes may be stable for a year or more, but may then fall off, over a period of time, to a lower level. The mechanical properties of parts of the electret, especially the mechanical dimensions of the diaphragm, may change with time.
It has been customary to pre-stress the diaphragm of an electret mechanically because appreciable stiffness is desirable for effective operation in either a microphone or an acoustic reproducer. If the diaphragm is too compliant, it may collapse against the other electrode. In most electret assemblies, conventional wisdom has required firm anchoring of the rim or periphery of the diaphragm, so that a tensioned condition can be maintained. A few prior constructions have utilized specialized diaphragm constructions that do not require pre-stressing of the diaphragm, as in Carlson et al. U.S. Pat. Nos. 3,740,496 an Sawyer 4,418,246. But those specialized diaphragm constructions have usually employed corrugations or "bumps" of one form or another, either at the rim or in the central portion of the diaphragm.